[Python-Dev] PEP 492: No new syntax is required

[Mark Shannon]

On 26/04/15 23:24, Nick Coghlan wrote:
>
> On 27 Apr 2015 07:50, "Mark Shannon" <mark at hotpy.org
> <mailto:mark at hotpy.org>> wrote:
>  > On 26/04/15 21:40, Yury Selivanov wrote:
>  >>
>  >> But it's hard.  Iterating through something asynchronously?  Write a
>  >> 'while True' loop.  Instead of 1 line you now have 5 or 6.  Want to
>  >> commit your database transaction?  Instead of 'async with' you will
>  >> write 'try..except..finally' block, with a very high probability to
>  >> introduce a bug, because you don't rollback or commit properly or
>  >> propagate exception.
>  >
>  > I don't see why you can't do transactions using a 'with' statement.
>
> Because you need to pass control back to the event loop from the
> *__exit__* method in order to wait for the commit/rollback operation
> without blocking the scheduler. The "with (yield from cm())" formulation
> doesn't allow either __enter__ *or* __exit__ to suspend the coroutine to
> wait for IO, so you have to do the IO up front and return a fully
> synchronous (but still non-blocking) CM as the result.
True. The 'with' statement cannot support this use case, but
try-except can do the job:

trans = yield from db_conn.transaction()
try:
     ...
except:
     yield from trans.roll_back()
     raise
yield from trans.commit()

Admittedly not as elegant as the 'with' statement, but perfectly readable.

>
> We knew about these problems going into PEP 3156
> (http://python-notes.curiousefficiency.org/en/latest/pep_ideas/async_programming.html#using-special-methods-in-explicitly-asynchronous-code)
> so it's mainly a matter of having enough experience with asyncio now to
> be able to suggest specific syntactic sugar to make the right way and
> the easy way the same way.
asyncio is just one module amongst thousands, does it really justify 
special syntax?

Cheers,
Mark.